Evolutionary history, phylogenetic diversity (PD) and tipping points

Evolutionary history and phylogenetic diversity (PD) are appropriate targets for biodiversity conservation because they represent many current and future benefits for humans (evolutionary or “evosystem services”; see Faith et al 2010). These benefits are reduced when branches of the phylogenetic tree are lost through extinctions. Successive species extinctions each may imply only a moderate loss of evosystem services, until, abruptly, the last descendent species from a long branch goes extinct – and the long branch representing a large amount of PD is now lost. Phylogenetic risk analysis can guide conservation decisions that try to reduce risk of these worst case losses, or “tipping point” outcomes (Faith et al 2010).

We have explored such tipping points for the taxonomic group that is perhaps most threatened by climate and land use changes – the corals. Carpenter et al. (Science, 2008) calculated that “32.8% of zooxanthellate corals fall into threatened categories, compared to approximately 25% of mammals and 14% of birds...If Near Threatened species are added, the proportion of corals (57.8%) exceeds that of all terrestrial animal groups assessed to date...”

These impacts may be even greater at the level of evolutionary history. Based on a large phylogenetic tree for corals, we found that threats are well-dispersed on the phylogenetic tree (lots of PD would still persist). However, there are many examples where entire monophyletic groups (existing families and genera) fall into IUCN threatened (or near-threatened) classes. We identified several cases representing potential tipping points and loss of resilience. For example, all listed species within the genera Catalaphyllia, Physogyra, and Euphyllia are listed in one of the threatened or near-threatened categories. The phylogenetic tree for corals shows that these are the only descendent taxa of a long branch. These cases therefore provide notable examples of the potential loss of future benefits, and resilience, due to climate change impacts.

Overall, corals present relatively few examples of these phylogenetic tipping points – one reason is that the taxa most threatened by climate change are well-dispersed over the phylogenetic tree. At this stage, most studies for other taxonomic groups seem to suggest similar relatively small PD losses, given dispersed climate change impacts over phylogenetic trees (see Yesson and Culham, BMC Evolutionary Biology, 2006; Thuiller et al Nature, 2011; but also see Willis et al PNAS 2008).